Investment casting techniques innovated in the late 1960s have revolutionized the design, construction and performance of golf clubheads up to the present time Initially only novelty putters and irons were investment cast, and it was only until the early years of the 1980s that investment cast metal woods achieved any degree of commercial success. The initial iron clubheads that were investment cast in the very late 1960s and early 1970s innovated the cavity backed clubheads made possible by investment casting which enabled the molder and tool designer to form rather severe surface changes in the tooling that were not possible in prior manufacturing techniques for irons which were predominantly at that time forgings. The forging technology was expensive because of the repetition of forging impacts and the necessity for progressive tooling that rendered the forging process considerably more expensive than the investment casting process and that distinction is true today although there have been recent techniques in forging technology to increase the severity of surface contours able them at considerable expense.
The investment casting process, sometimes known as the lost wax process, permits the casting of complex shapes found beneficial in golf club technology, because the ceramic material of the mold is formed by dipping a wax master impression repeatedly into a ceramic slurry with drying periods in-between and with a silica coating that permits undercutting and abrupt surface changes almost without limitation since the wax is melted from the interior of the ceramic mold after complete hardening.
This process was adopted in the 1980s to manufacture "wooden" clubheads and was found particularly successful because the construction of these heads requires interior undercuts and thin walls because of their stainless steel construction. The metal wood clubhead, in order to conform to commonly acceptable clubhead weights on the order of 195 to 210 grams when constructed of stainless steel, must have extremely thin wall thicknesses on the order of 0.020 to 0.070 inches on the perimeter walls to a maximum of 0.125 inches on the forward wall which is the ball striking surface. This ball striking surface, even utilizing a high strength stainless steel such as 17-4, without reinforcement, must have a thickness of at least 0.125 inches to maintain its structural integrity for the high clubhead speed player of today who not uncommonly has speeds in the range of 100 to 150 feet per second at ball impact.
Faced with this dilemma of manufacturing a clubhead of adequate strength while limiting the weight of the clubhead in a driving metal wood in the range of 195 to 210 grams, designers have found it difficult to increase the perimeter weighting effect of the clubhead.
In an iron club, perimeter weighting is an easier task because for a given swing weight, iron clubheads can be considerably heavier than metal woods because the iron shafts are shorter. So attempts to increase perimeter weighting over the past decade has been more successful in irons than "wooden" clubheads. Since the innovation of investment casting in iron technology in the late 1960s, this technique has been utilized to increase the perimeter weighting of the clubhead or more particularly a redistribution of the weight of the head itself away from the hitting area to the perimeter around the hitting area, usually by providing a perimeter wall extending rearwardly from the face that results in a rear cavity behind the ball striking area. Such a clubhead configuration has been found over the last two plus decades to enable the average golfer, as well as the professional, to realize a more forgiving hitting area and by that we mean that somewhat off-enter hits from the geometric face of the club results in shots substantially the same as those hits on the geometric center of the club. Today it is not uncommon to find a majority of professional golfers playing in any tournament with investment cast perimeter weighted irons confirming the validity of this perimeter weighting technology.
Metal woods by definition are perimeter weighted because in order to achieve the weight limitation of the clubhead described above with stainless steel materials, it is necessary to construct the walls of the clubhead very thin which necessarily produces a shell-type construction where the rearwardly extending wall extends from the perimeter of the forward ball striking wall, and this results in an inherently perimeter weighted club, not by design but by a logical requirement.
In the Raymont, U.S. Pat. No. 3,847,399 issued Nov. 12, 1974, assigned to the assignee of the present invention, a system is disclosed for increasing the perimeter weighting effect of a golf club by a pattern of reinforcing elements in the ball striking area that permits the ball striking area to be lighter than normal, enabling the designer to utilize that weight saved on the forward face by adding it to the perimeter wall and thereby enhancing perimeter weighting.
This technique devised by Mr. Raymont was adopted in the late 1980s by many tool designers of investment cast metal woods to increase the strength of the forward face of the metal woods to maintain the requirement for total overall head weight and to redistribute the weight to the relatively thin investment cast perimeter walls permitting these walls to not only have greater structural integrity and provide easier molding and less rejects, but also to enhance the perimeter weighting of these metal woods. Most major companies in the golf industry manufacturing metal woods in the late 1980s were licensed under the Raymont patent.
In 1991, the Allen. U.S. Pat. No. 5,060,951 issued entitled "Metal Headed Golf Club With Enlarged Face", also assigned to the assignee of the present invention, and it discloses an investment cast metal wood with an enlarged club face depth (height) on the order of at least 1.625 inches. Such a face depth was not formerly believed possible because of the requirement for face structural integrity under the high impact loads at 100 to 150 feet per second, and the weight requirements of the clubhead of 195 to 210 grams. In this Allen patent, a labyrinth of reinforcing elements similar to Mr. Raymont's was utilized not to re-distribute face weight but instead to enlarge face area while maintaining overall clubhead weight. An ancillary and important advantage of this development, utilized by many present day designers of "jumbo" metal wood heads, is the fact that an enlarged club face produces a sweet spot enlargement far greater than the enlargement of the club face itself.
There are however limitations on the effectiveness of the reinforcing elements on the face wall of investment cast clubs and particularly metal woods. Because investment cast metal woods must have hollow interiors, these interiors must be formed by removable core pieces. To the present day face wall reinforcement has been effected in accordance with the above Raymont and Allen patents by forming integral ribs and bars on the rear surface of the forward ball striking wall. In order to effect this rib pattern, the core pieces that form the rear surface of the ball striking wall, as well as the ribs themselves, must be withdrawn rearwardly in order to clear the ribs. However, the perimeter wall extending rearwardly from the forward wall inhibits the direct rearward removal of these core pieces from the forward wall during the casting operation. Therefore, it has been commonplace to either make these reinforcing elements very shallow on the order of 0.030 to 0.050 inches in rearward depth or to rearwardly taper the ribs almost to a point extending rearwardly from the forward face so that these core pieces can move laterally somewhat as they are removed from the forward wall at the completion of the casting cycle.
These limitations detract from the effectiveness of the reinforcing elements and their capability of achieving a lighter front ball striking wall. As described in the Raymont patent, the effectiveness of the reinforcement of the forward wall is determined by the "I" or "T" beam configuration of the reinforcing elements. The amount of reinforcement is determined in part by the depth and width of the reinforcing walls in a plane transverse of the ball striking wall at its point furthest from the ball striking wall. In an "I" beam configuration, the width of the cross piece away from the forward wall, can be selected as desired but is extremely difficult to mold because of the undercut on the rear web. Such increase in web width and augmentation of the depth of the reinforcement has not to this date been possible prior to the present invention, and hence the full advantages of increased perimeter weighting, superior face reinforcement, and face enlargement have not been thus far fully exploited.
Another problem addressed by the present invention is the achievement of increasing the benefits of perimeter weighting by simply adding weight to the perimeter of the clubhead itself. This technique of course has found considerable success in low impact clubheads such as putters, where overall clubhead weight is in no way critical, and in fact in many low impact clubs that have found considerable commercial success, the clubheads weigh many times that of metal wood heads, sometimes three or four times as heavy.
To this date, however, increased perimeter weighting has not been found easy because of the weight and impact strength requirements in metal woods. An understanding of perimeter weighting must necessarily include a discussion of the parameter radius of gyration. The radius of gyration in a golf clubhead is defined as the radius from the geometric or ball striking axis of the club along the club face to points of clubhead mass under consideration. Thus in effect the radius of gyration is the moment arm or torquing arm for a given mass under consideration about the ball striking point. The total moments acting on the ball during impact is defined as the sum of the individual masses multiplied by their moment arms or radii of gyration. And this sum of the moments can be increased then by either increasing the length of the individual moment arms or by increasing the mass or force acting at that moment arm or combinations of the two.
Since it is not practical, except for the techniques discussed in the above Raymont and Allen patents, to add weight to the perimeter wall because or the weight limitations of metal woods and particularly the driving woods, one alternative is to increase the moment arm or radius of gyration. This explains the popularity of today's "jumbo" woods although many of such woods do not have enlarged faces because of the requirement for structural integrity in the front face.
Another problem arises from the aerodynamics of today's metal woods as well as those of the "wooden" type. The top wall in many metal and wooden woods has an aerodynamic shape but due lo the configuration of the sole plate and the back wall, there is no possible air foil lift generated in the normal clubhead impact speed range of 100 to 150 feet per second. In fact, there can be a negative lift or downward drag on the clubhead as the head moves through the hitting area due to the fact that the length of the air stream passing under the clubhead is greater than the length of the air stream passing over the top wall because the sum of the length of the sole plate and back wall in a vertical plane passing down the target line through the clubhead is greater than the length of the top wall in the same plane. Applying the law of continuity to these parameters results in the air stream along the bottom of the clubhead having a lower pressure than the air stream passing along the top of the clubhead and hence a resulting downward force on the clubhead as it passes through the hitting area at high speed.
It is a plurality object of the present invention to ameliorate the problems of interior face reinforcement, increasing the radius of gyration, and improving the aerodynamic characteristics of a high impact golf clubhead.